Examples of GRB Localizations

To give an idea of how the localization technique works, we report some examples of GRBs, whose direction was already known with moderate ($\sim$ few degrees, like in the case of BATSE) and high accuracy ($\sim$ few arcminutes, like for the WFC or IPN bursts) thanks to other instruments, that detected them as well.

Figure: GRBM Localization of GRB980109: the different $\chi^2$ levels are shown; The triangle denotes the burst position as determined by the WFC, while the asterisk spots the GRBM centroid.

In figg.  and , some examples of GRB localizations are shown: the frame of reference is local to BeppoSAX (an aitoff projection has been used): fixed the elevation coordinate $\theta = 0$, the azimuthal coordinate $\phi$ assumes the values 0, 90, 180 and 270 (-90 in the figure), when the direction faces the GRBM unit 2, 3, 4 and 1, respectively. Both cases concern bursts, whose direction has been measured with high precision: GRB980109 was localized by the WFC2 at coordinates R.A.= 0h 25m 56s, Decl. = $-63\rm ^{\circ}01\mbox{$^\prime$}.4$ (2000.0; in local coordinates: $\phi=90.6\rm ^{\circ}$, $\theta=+12.0\rm ^{\circ}$), with a $10\mbox{$^\prime$}$ error radius (IAUC. 6805); this burst triggered BATSE as well (trig. number: 6564). GRB00516 was localized by means of an IPN triangulation (GCN 671), thanks to Ulysses, BeppoSAX/GRBM, Konus/WIND and NEAR: its coordinates were found to be R.A.= 2h 20 m 16.13 s, Decl.= $-14\rm ^{\circ}50\mbox{$^\prime$} 14.33\mbox{$^{\prime\prime}$}$ (locally: $\phi=356.6\rm ^{\circ}$, $\theta=+25.0\rm ^{\circ}$) with a $3\sigma$ error box area of $\sim 50$ square arcminutes.

Figure: GRBM Localization of GRB000516: the different $\chi^2$ levels are shown; The triangle denotes the burst position as determined by the IPN, while the asterisk spots the GRBM centroid.

Since the GRBM response matrix has been used to estimate the arrival directions for about two hundreds GRBs detected with the only GRBM, for which, therefore, there is no possibility to check the accuracy of the localization with other experiments, it is fundamental to know the systematic error and, in general, the limits introduced by this technique. On this subject a detailed discussion will follow in the next chapters, when dealing with the results from the sample of bursts that were localized by both the GRBM and other experiments. These samples of common bursts have been used to calibrate this technique and to test it. Here we limit our attention to some remarks: first, only for a subset of GRBs identified with the search algorithms, which are the subject of this thesis, the localization has come out to be meaningful: e.g., when the burst is faint, its total counts in the GRBM units are affected by low precision, and this poor knowledge turns into several and wide error boxes in the sky, making the localization useless. For this and other similar reasons, when dealing with the localizations of the bursts described in this catalog, several conditions had to be fulfilled in order to give a unique and meaningful solution: this choice limited the number of bursts, for which it was possible to estimate the incoming direction. For instance, another (obvious) condition was that the candidate direction, together with its whole error box, must be at least partially visible (i.e., not Earth-blocked) from the BeppoSAX point of view. For a detailed discussion of these criteria and of their related selection effects on the sample of bursts localized with the GRBM, see section .

Figure: GRBM Localization of GRB990627: the different $\chi^2$ levels are shown; this localization shows different relative minima.

An example of multiple solution for the direction of the same burst is given by GRB990627, UT 04:00:25 (fig. ). This burst, missed by the on-board logic trigger, was quite bright on the GRBM unit 3; the GRBM $\chi^2$ map for this burst suggests three possible directions: two, pretty symmetrical, respectively above and below the axis direction of unit 3, and, therefore, quite close to the WFC 1 field of view ( $\phi=-90\rm ^{\circ}$); the third one is found at $\phi \sim 90\rm ^{\circ}$ and $\theta \sim 50\rm ^{\circ}$. Due to this ambiguity, this burst has not fulfilled all the requirements for an acceptable localization, and for this it has not been included in the sample of GRBM localized bursts.